The mobile communication system has evolved into a high-speed, high-quality wireless packet data communication system to provide data and multimedia services beyond the early voice-oriented services. Recently, various mobile communication standards, such as High Speed Downlink Packet Access (HSDPA), High Speed Uplink Packet Access (HSUPA), Long Term Evolution (LTE), and LTE-Advanced (LTE-A) defined in 3rd Generation Partnership Project (3GPP), High Rate Packet Data (HRPD) defined in 3rd Generation Partnership Project-2 (3GPP2), and Institute of Electrical and Electronic Engineers (IEEE) 802.16, have been developed to support the high-speed, high-quality wireless packet data communication services. Particularly, LTE is a communication standard developed to support high speed packet data transmission and to maximize the throughput of the radio communication system with various radio access technologies. LTE-A is the evolved version of LTE to improve the data transmission capability.
As one of the broadband mobile communication systems, the LTE system adopts Orthogonal Frequency Division Multiplexing (OFDM) scheme in the downlink and Single Carrier Frequency Division Multiple Access (SC-FDMA) in the uplink. Such multiple access schemes allocate and manage the time-frequency resources for transmitting user-specific data or control information without overlap, i.e. maintaining orthogonality, so as to differentiate user-specific data and control informations.
FIG. 1 is a diagram illustrating a structure of the radio resource in downlink of the LTE and LTE-A system according to the related art.
FIG. 1 shows the basic structure of time-frequency grid of radio resource mapping relationship between the downlink physical channel and signals for transmitting data and control channels in the downlink of an LTE/LTE-A system.
Referring to FIG. 1, the horizontal axis denotes time, and the vertical axis denotes frequency. The smallest unit of transmission in time domain is OFDM symbol 104, and a slot 101 consists of NsymbDL OFDM symbols (typically, NsymbDL=7). A subframe 102 spans 1 ms and consists of two 0.5 ms slots, and a radio frame 103 spans 10 ms and consists of 20 slots, i.e. 10 subframes. The smallest unit of transmission in frequency domain is subcarrier 105, and the entire system bandwidth 109 consists of total NBW subcarriers. Here, NBW is in proportion to the system bandwidth. In the time-frequency grid, Resource Element (RE) 106 is the basic unit indicated by OFDM symbol index and subcarrier index. The Resource Block (RB) or Physical Resource Block (PRB) 108 consists of NsymbDL consecutive OFDM symbols in the time domain and NscRB consecutive subcarriers (Typically NscRB=12) in the frequency domain. Accordingly, one RB consists of NsymbDL×NscRB REs. Typically, an RB is the smallest unit of resource that can be allocated to a user.
The downlink control channels are transmitted in the first N OFDM symbols at the beginning of a subframe. Typically, N={1, 2, 3}. N varies depending on the amount of the control information sent at every subframe. The control channels 110 include Physical Control Format Indicator Channel (PCFICH) carrying an indicator indicating the value N, Physical Downlink Control Channel (PDCCH) carrying uplink or downlink scheduling information, and Physical Hybrid Automatic Repeat reQuest (HARQ) Indicator Channel (PHICH) carrying HARQ ACKnowledgement (ACK)/Non-ACKnowledgement (NACK). The Physical Downlink Shared Channel (PDSCH) 111 is transmitted at the region where no downlink control channels are mapped.
An evolved Node B (eNB) transmits Reference Signal (RS) for use in downlink channel state measurement or PDSCH decoding at a User Equipment (UE). The RS is a pilot signal. The RS may be categorized into one of Cell-specific Reference Signal (CRS) 112 which all UEs can receive within the cell, Channel Status Information Reference Signal (CSI-RS) 114 transmitted on different antenna ports using a smaller amount of resource as compared to CRS, and DeModulation Reference Signal (DM-RS) 113 referenced to decode PDSCH scheduled to a predetermined UE.
The CSI-RS is defined per antenna port for use in measuring channel status of each antenna port. If the same CSI-RS is transmitted through multiple physical antennas, the UE cannot discriminate among the physical antennas but recognizes a single antenna port.
The eNB may transmit CSI-RS at cell-specific positions. The reason for assigning the time and frequency resources at cell specific positions is to prevent inter-cell interference of CSI-RS.
In the LTE/LTE-A system, in order for the UE to acquire cell ID and subframe and radio frame synchronization and frequency synchronization, the eNB transmits Primary Synchronization Signal (PSS) and Secondary Synchronization Signal (SSS). The eNB uses predetermined sequences for the respective PSS and SSS which are repeatedly transmitted at predetermined positions in a radio frame.
FIG. 2 is a diagram illustrating the positions of PSS and SSS in a radio frame of LTE/LTE-A Frequency Division Duplex (FDD) system according to the related art.
Referring to FIG. 2, the PSS is transmitted at OFDM symbol#6 201 and 203 of subframe#0 and subframe#5 in the time domain. Meanwhile, the SSS is transmitted at OFDM symbol#5 202 and 204 of subframe#0 and subframe#5. In the frequency domain, the PSS and SSS are mapped to 6 RBs 205 of the system bandwidth.
The UE uses PSS and SSS for tracking the time and frequency of the corresponding cell continuously and detects and measures PSS and SSS of neighboring cells as preparation for handover to any neighboring cell.
Recently, there are many discussions on how to reduce system overhead to improve energy efficiency of LTE/LTE-A system. Such discussions include minimizing downlink control channel transmission and reducing transmission interval of CRS in the time-frequency domain.
The above information is presented as background information only to assist with an understanding of the present disclosure. No determination has been made, and no assertion is made, as to whether any of the above might be applicable as prior art with regard to the present disclosure.